Quality Scan: High Cpk Holemaking
Honing is one of the few metalworking processes where manually operated machines, whether new or decades old, do yeoman service in millions of shops every day on basic bore sizing and finishing applications. At the other end of the technology spectrum, today's most advanced CNC-controlled hones are among the rare machines capable of automatically controlling hole size to accuracies of 0.25 µm, with minimal variability and no operator intervention.
Accuracy and process stability have led to a renaissance for honing, particularly for parts produced to high-Cpk requirements. Makers of gears, hydraulic valves, and small engines, to name just a few products, have discovered automated honing in their drive to make parts with tolerances as tight as ±.0002" (±5 µm) at Cpk levels of 2.5. As a result, gears run quieter, smoother, and longer; hydraulic valves are more efficient and leak-resistant; and small engines burn fuel more efficiently.
Various holemaking processes, such as boring, drilling, and reaming, are capable of holding good tolerances, but when a high Cpk requirement is imposed, it changes the picture entirely. For rule-of-thumb purposes, when the target is 1.33 Cpk, manufacturers find they have to hold about 60% of the print tolerance; at 1.67 Cpk, the requirement becomes about 40% of tolerance. Holes produced satisfactorily on a lathe for years that suddenly have to meet process capability of 1.33 or 1.67 Cpk may require as much narrower bell curve distribution. Flyers at the fringes of the curve become unacceptable.
Why does a high Cpk constrict the tolerance band? Remember that Cpk is calculated as upper tolerance minus the mean or the mean minus the lower tolerance, whichever is smaller, and this value is divided by three times the standard deviation. A stable, consistent process helps keep the standard deviation in the denominator small. If the high mean of the group can be focused exactly in the middle of the tolerance range, it helps produce the largest numerator.
To get that large numerator and small denominator, you not only want a process where the variability is small, you must have a process where you can accurately target your mean to a certain value easily and predictably. A lathe may get to a certain point, but tweak it a little and it jumps to a value out of spec. Honing, especially when done on a computer-controlled hone, can easily get within 10 millionths of a specified size, and with the resolution on the feed systems of today's machines, the variability is very small.
The latest generation of machines introduced at IMTS 2006 combines a patent-pending tool-feed system with an integrated post-process air-gaging system. Combining an airgaging system and tool-feed control eliminates the need for an experienced honing operator to tweak the process. The new air-gaging system takes post-process measurements of the parts while they are still fixtured on the machine's rotary table, resulting in very high accuracy for tool-feed control.
In-process air gaging integrated in the honing tool has been around for a few decades, but it's best used for automatic shut-off.The post-process system produces the greater accuracy needed for tool-size control when working to high Cpk standards. It eliminates measurement uncertainties caused by an undersized or worn gage probe, which can occur with a hone-head air gage. It also allows measurement without interference from swarf and oil.
This servo-controlled machine knows precisely where the tool is, and how much it has been fed. Some of the older honing machines fed the tool based on force—the machine knew how hard it was pushing, but it didn't know tool size at any given time. These new machines do. With CGT Krossgrinding tools, size can be adjusted with a resolution of 0.025 µm. On these tools, the diamond-plated section wears so little that they may produce thousands of parts before size compensation is needed.
Any operator with multiaxis CNC experience will find these new machines familiar. The servo-controlled stroke system ensures a consistent crosshatch pattern, and can dwell in any part of the hole, end-to-end.The machine can even make corrections that are not intuitive for an operator. Switchable control features, such as correct for bore shape, allow the operator to select a problem bore image, such as barrel or taper, and the machine will automatically correct the part. Combined with feedback from air gaging of finished parts, this honing system can eliminate the operator's art from precision bore sizing.
This article was first published in the February 2007 edition of Manufacturing Engineering magazine.